Dealing with missingness

[chadhazlett]

Ideally we'd deal with missingness much as lm does -- i.e. it is na.omitted internally but then the predictions etc. are put back into the full length vector with the NA intermixed.

[lukesonnet]

What about in the returned K, U, etc?

[chadhazlett]

I would propose we leave out the missingness in those objects and just keep NAs in all the fit/predict type objects. Main motive for this would be that if people are generating predictions by multiple procedures then it needs to be easy to weave them together.